The invention relates to the field of human survivability in elevated temperature situations such as fire-fighting or chemical warfare situations involving contamination suits in elevated temperatures. It is also useful for reduced temperature operation such as in the polar regions or during underwater diving operations where heating of the breathing gas is necessary.
Existing applications of self-contained breathing apparatus gas circuits, either closed circuit or open circuit, suffer from deficiencies in the thermal aspects of their operation. In a fire, heat gain from radiation and convection in the breathing gas circuit can push inhalation temperatures to critical temperatures (above 120 degrees for wet gases and 180 degrees for dry gases) thereby endangering the life of the fire fighter. Additionally, closed circuit systems which use a chemical scrubbing canister suffer from high heat gain from the canister alone. In bulky chemical warfare suits, although inhalation gas temperatures are not life threatening, they cause psychological stress, particularly claustrophobia. For reduced temperature operations such as in underwater diving, either hot water is pumped to a heat, exchanger to warm the breathing gas, or the diver must suffer with the existing breathing gas temperature until it becomes unbearable.
At the current time, design of breathing gas circuits for use in elevated temperatures consist of simple insulation schemes in an attempt to minimize heat gains. Safe operation relies on the premise that fire fighters will exit the fire due to heat stress or the high temperature of the breathing gas when appropriate. Breathing gas circuits in particular are not optimized for the complex thermal situations encountered in fire fighting situations. These situations are complex, because the thermal circuit needs to be designed differently for the two major scenarios encountered in fire fighting. The first scenario involves operation in elevated ambient temperatures, in which heat will flow into the circuit, making it desirable to have the circuit well insulated to prevent the absorption of heat. The second scenario is entry to and exit from the fire situation, when the ambient temperature is less than that of the breathing gas circuit, making it desirable to have an uninsulated system to allow the heat to flow out of the breathing gas circuit. Because of this conflicting thermal requirement, which is aggravated in the case of closed circuit systems which have the additional heat gain from the canister, existing breathing gas systems are generally not optimized at all or have simplistic insulation schemes.
Existing cooling schemes for high temperature operation generally consist of using ice packs in a chest vest to provide core temperature cooling. Usage of ice pack technology on board Navy ships is undesirable due to the fact that the Navy can not rely on ice being available during combat damage control situations.